An image sensor typically comprises a two-dimensional array of photo detector circuits. A photo detector circuit may provide a detection quantity having a level that depends on an amount of light to which the photo detector circuit has been exposed during a photo detection time interval. A front-end circuit receives the detection quantity that the photo detector circuit provides. In response, the front-end circuit provides an output signal indicative of the amount of light.
The front-end circuit defines a relationship between the output signal and the level of the detection quantity. For example, the front-end circuit may provide a pulse having a width, or rather duration, that depends on the level of the detection quantity. A given level of magnitude is converted into a given duration of time. The output signal of the front-end circuit may be processed so as to obtain, for example, a pixel value. For example, the width of the aforementioned pulse may be converted into a digital value by means of a counter.
The article entitled “A 0.5 V Sub-Microwatt CMOS Image Sensor with Pulse-Width Modulation Read-Out” by Scott Hanson et al., published in the IEEE Journal of Solid-State Circuits, Vol. 45, No. 4, April 2010, describes an image sensor with pulse-width modulation read-out. In this image sensor, a pixel structure comprises a two-transistor comparator. This comparator constitutes a front-end circuit, which is coupled to a photo detector circuit as described hereinbefore.